Wind turbine blade

ABSTRACT

A turbine blade ( 10 ) and turbine blade arrangement ( 5 ) for extracting energy from a moving fluid are described. A Savonius-type rotor assembly incorporates a turbine blade ( 10 ) and/or a turbine blade arrangement ( 5 ) where the blades ( 10, 10 ′) may be twisted to generally form a helix. The cross sectional profile for the blades ( 10, 10 ′) may represent an airfoil, and may be asymmetric. In some embodiments, the cross sectional profile remains substantially constant over the length of each blade ( 10,10 ′). Turbines utilizing the described blade ( 10 ) and blade arrangement ( 5 ) may be self starting and may have a relatively smooth torque profile throughout the entire rotational path. The blades ( 10, 10 ′) and blade arrangement ( 5 ) may extract energy from a moving fluid through both lift and drag forces.

RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No. 60/943,623, filed Jun. 13, 2007, which is fully incorporated by reference herein.

BACKGROUND

The field of the present disclosure generally relates to wind turbines, and turbine blade designs. Throughout this description the term “wind turbine” is used. However, the term “wind turbine” is not limited to turbines moved by the wind, but includes turbines rotated by any moving fluid.

With rising concerns over global warming and increased demands for power, much emphasis has been placed on generating clean energy. Wind machines have been used for centuries, but recently have been the subject of enhanced efforts to improve efficiency, usability and durability.

Modern wind turbines, specifically large horizontal axis turbines resembling aircraft propellers, are based on airfoil design principles to increase the amount of power harvested from the wind. Large horizontal axis wind turbines operate effectively in moderate winds, and are efficient at capturing energy, that is, a relatively high percentage of the fluid energy is converted to mechanical or electrical energy, or both. However, horizontal axis wind turbines, in order to efficiently capture energy from the wind, require blade lengths ranging between approximately 20 to 70 meters, which in turn requires large tracts of land for installation. Due to their large size, horizontal axis wind turbines, which can be over 100 meters tall, are mechanically complex and can be dangerous to operate near populated areas because the long blades are subject to high speeds and large stresses, and occasionally, fracture and break.

One type of a mechanically less complex wind turbine design is the Savonius-type turbine. Savonius-type turbines have two or more elongate blades rotating about an axis that may be horizontally or vertically aligned. For example, U.S. Pat. No. 7,132,760 describes a Savonius-type wind turbine where the axis may be vertically or horizontally aligned. Instead of the blades radiating from a hub like an aircraft propeller, Savonius-type turbine blades are arranged so that the long axis of each blade generally extends along the axis of blade rotation.

Savonius-type wind turbines started out being pure drag machines, that is, operating due only to differential drag on the curved, elongate blades. When wind encounters a Savonius-type blade set, the “cupping” effect of the wind on the concave side of one blade produces greater drag than the impact of the wind on the convex side of another blade. The greater drag from the “cupped” wind causes the blade set to rotate and is used to harness power from the wind, albeit inefficiently.

Compared to large horizontal axis wind turbines, Savonius-type wind turbines are relatively small, are mechanically simpler, and operate in a broad spectrum of wind speeds—for example, but not limited to, 1 to 4 meters per second—to heavier winds—for example 45 meters per second and higher. Savonius-type wind turbines are therefore well suited for use near and in populated areas, as well as remote locations.

Attempts to improve traditional Savonius-type wind turbines have been made. For example, U.S. Pat. No. 5,494,407 discloses non-twisted blades with a cross section that is curved then straight. U.S. Pat. No. 4,784,568 discloses non-twisted blades with a cross section resembling the cross section of a wing from a light aircraft. U.S. Pat. Pub. No. 2007/0104582 discloses non-twisted blades with a complexly curved cross section that is a computerized optimization to increase torque output from the blades depicted in the '568 patent. While each of these non-twisted blade designs attempts to increase the amount of power extracted from the wind, they exhibit a differential wind load which causes a pulsating torque.

The differential wind load is due to the fact that the entire length of each non-twisted blade alternately moves into and out of the wind as the blade assembly rotates. That is, each blade moves through four distinct positions as it rotates. The distinct positions are (1) fully broadside to the wind path producing resistance that contributes to torque, or “cupping,” (2) a lee position where the blade is edgewise to the wind direction and substantially out of the wind, (3) fully broadside to the wind path producing resistance that counters torque, and (4) a windward position where the blade is edgewise to the wind direction and substantially in the wind. The differential wind loading on the blade as it rotates introduces additional vibrations and stresses to wind turbine components such as the blades themselves and the bearings supporting the rotor assembly. Additional vibrations and stresses reduce wind turbine efficiency and durability—often leading to rotor bearing failure.

Other attempts to improve Savonius-type wind turbines employ twisted blades. Twisted blades present a substantially constant surface area to the wind as the rotor assembly turns and therefore exhibit a lesser pulsing torque than the previously described non-twisted blades. However, twisted blades are complex to fabricate as they have a twist along a longitudinal axis. Many twisted blades are made from a flat material which is twisted into a helix. Other blades exhibit a curved cross section which is generally symmetric about the longitudinal axis before a twist is introduced along the longitudinal axis. Some devices, such as U.S. Pat. No. 7,132,760 for example, include a set of Darius-type airfoil cross sectional blades with a separate set of twisted Savonius-type blades. However, the non-twisted blades introduce a pulsating torque profile and its attendant vibration as described above.

Thus, the present inventor has recognized a need for a Savonius-type wind turbine with improved efficiency blades, and one with a smooth torque profile.

SUMMARY

The below description is directed to improved wind turbines and turbine blade design. Certain preferred embodiments are disclosed that provide a technical contribution by more efficiently harnessing power from the wind, or other moving fluid, while maintaining a smooth torque profile that does not introduce substantial vibration or stress to turbine components.

Additional aspects and advantages will be apparent from the following detailed description of preferred embodiments, which proceeds with reference to the accompanying drawings.

Additional aspects and advantages will be apparent from the following detailed description of preferred embodiments, which proceeds with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view of a preferred embodiment taken at the surface of a rotor assembly plate.

FIG. 2 is a side perspective view of a wind turbine blade configuration having blades with the cross section of FIG. 1.

FIG. 3 is a side perspective view of the wind turbine blade configuration of FIG. 2 with the blades rotated about a central axis by 90 degrees.

FIG. 4 is an enlarged view of the cross-sectional profile for blade 10 illustrated in FIG. 1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Embodiments discussed below may address and solve certain problems related to harnessing power from a moving fluid using a Savonius-type turbine with high efficiency and a smooth torque profile. One with ordinary skill in the art will realize that the following discussion is illustrative and intended to describe preferred embodiments and is not intended to limit the present invention to the embodiments discussed. The described embodiments, as well as other embodiments, have numerous applications where a wind, or other fluid, turbine is employed, and may be scaled and adapted to many applications in view of the description that follows.

The above needs, and others, may be overcome by employing wind turbine blades with asymmetric cross sections that are also formed into a helix along the length of the blade. In certain embodiments, the cross sectional profile remains uniform over the length of the blade.

A preferred embodiment is formed using two blades held between two rotor plates. Each blade has an asymmetric cross sectional profile defining a leading edge first curved section, for example a convex curve, with a first radius. A substantially linear intermediate section is tangent to the first section, and is also tangent to a trailing edge second curved section. The second curved section has a radius which is less than the radius of the first curved section. A longitudinal, generally helical twist is preferably 180 degrees from end to end, and both the leading edge and the trailing edge exhibit the generally helical twist. The leading edge of each blade lies on an opposite end of a first diameter of a circle, and the trailing edge of each blade is located past a second diameter of the same circle which bisects and is orthogonal to the first diameter. Grooves in each of the rotor plates match the cross sectional profiles of the blades and hold the blades in place when the rotor plates are urged towards one another.

Referring now to the drawings, FIG. 1 illustrates a wind turbine blade arrangement 5 comprising turbine blades 10, 10′. The wind turbine blades 10, 10′ cross section is taken at the surface of rotor plate 200 showing the ends of the blades 10, 10′. In preferred embodiments, the cross section is substantially uniform throughout the length of each blade 10, 10′. While rotor plate 200 is illustrated as circular, embodiments are not limited to circular rotor plates as the rotor plate 200 may be any substantially symmetrical shape. FIGS. 2 and 3 illustrate two rotor plates in the preferred embodiment, however other embodiments may utilize only one rotor plate.

Due to the helical twist imparted to each blade 10, 10′, as the blade 10, 10′ extends in a direction away from the rotor plate 200, the cross sectional profile is rotated and moved across diameters 110 and 120. For example, following the curve for leading edge 21 extending from rotor plate 200 to rotor plate 300 (FIG. 2), leading edge 21 is rotated and moved to the same position as leading edge 21′(FIG. 1), but on rotor plate 300 (FIG. 2) instead of on rotor plate 200. Likewise, trailing edge 41 is rotated and moved across diameters 110 and 120 so that trailing edge 41 is located at the same position as trailing edge 41′, but on rotor plate 300 (FIG. 2) instead of rotor plate 200.

When wind approaches the blades 10, 10′ from directions generally between wind lines 500 and 501, and between 600 and 601, respectively, that is, substantially edge-wise, the wind encountering a leading edge 21 or 21′ on blades 10, 10′ creates a lifting, torque-producing force. The lifting, torque-producing force results from the cross-sectional profile of blades 10, 10′ acting as an airfoil when wind encounters leading edges 21, 21′ at certain angles of attack. Modifying the cross-sectional profile, for example, but not limited to, altering the curvature of sections 20, 20′ and 40, 40′, the distance between leading edges 21, 21′ and trailing edges 41, 41′, and the angular offset between the leading edge 21, 21′ and the trailing edge 41, 41′ (that is, the difference in the distance leading edge 21, 21′ is located from diameter 110 and the distance trailing edge 41, 41′ is located from diameter 110) may modify the angles of attack that produce a lifting, torque-producing force.

Edge-wise wind also encounters a trailing edge 41 or 41′ and creates a drag, counter-torque force opposite to the lifting, torque-producing force. Due to the cross sectional profile of blades 10, 10′, the lifting, torque-producing force is stronger than the drag, counter-torque force which causes the rotor assembly 5 to turn in a clockwise direction indicated by arrow “R.”

When wind approaches the blades 10, 10′ from directions generally between wind lines 500 and 600, and 501 and 601, respectively, that is, substantially broadside, wind encounters a front side 60 or 60′ and a back side 70 or 70′. The generally cup shaped front sides 60, 60′ create more drag, torque-producing force than the generally convex back sides 70, 70′, which create drag, counter-torque force. The greater drag, torque-producing force created by the front sides 60, 60′ causes the blades 10, 10′ to turn the rotor assembly 5 in a clockwise direction indicated by arrow “R.”

The previous discussion regarding torque-producing and counter-torque forces shows that at each vertical position of the blades 10, 10′, and regardless of wind direction, the location of blade 10 with respect to blade 10′ and the cross sectional profile of blades 10, 10′ provide a rotational force in the same rotational direction “R.”

Referring to FIGS. 2 and 3, the generally helical twist for blades 10, 10′ preferably rotates one end of a blade approximately 180 degrees, so that front side 60 or 60′ faces in one direction at one end of blade 10 or 10′, and in an opposite direction at the other end of blade 10 or 10′ as seen in FIG. 2, for example. The generally helical twist also longitudinally offsets the two ends of blades 10, 10′ so that when viewed from direction “V” (FIG. 1), the two blades 10, 10′ form a general shape of an “X” (FIG. 2). The degree of twist and blade end offset may be modified, for example to accommodate a different number of blades, fluid density or prevailing fluid speed.

Referring to FIG. 2 and taking for example wind blowing along direction “V” (FIG. 1), it is apparent that wind from any one direction encounters different angular orientations for each blade 10, 10′ at various vertical positions between rotor plates 200 and 300. Fluid moving between rotor plates 200 and 300 therefore impacts both front sides 60 and 60′ at the same time because blades 10, 10′ are preferably twisted through 180 degrees. It is also apparent that fluid from any one direction impacts both back sides 70 and 70′, as well as leading edges 21 and 21′ and trailing edges 41 and 41′. Each surface and edge 60, 60′, 70, 70′, 21, 21′, 41 and 41′ contribute torque-producing and counter-torque forces substantially as described above.

Referring to FIGS. 2 and 3, it is apparent that because each blade 10, 10′ is rotated through approximately 180 degrees in a longitudinal direction, that is between rotor plates 200 and 300, an approximately equal amount, or percentage, of front sides 60, 60′, back sides 70, 70′, leading edges 21, 21′ and trailing edges 41, 41′ encounter the oncoming wind at all times. Because the amount, or percentage, of front sides 60, 60′, back sides 70, 70′, leading edges 21, 21′ and trailing edges 41, 41′ encountering the oncoming wind remains substantially constant as rotor assembly 5 rotates in direction “R” (FIG. 1), the amount of torque-producing and counter-torque forces created by blades 10, 10′ remains substantially constant (assuming a relatively constant fluid flow).

The location of where the torque-producing and counter-torque forces are generated changes longitudinally between rotor plates 200 and 300. For example, and assuming wind blowing in direction “V” (FIG. 1), more torque-producing drag is produced at the ends of blades 10, 10′ in FIG. 2, whereas more torque-producing drag is produced at the middle of blade 10′ in FIG. 3. However, because the amount of torque-producing force remains substantially constant (at a given fluid speed), and because dual forces of lift and drag contribute to the torque-producing forces, the longitudinal transition is relatively smooth and does not introduce substantial vibrations to the wind turbine. Because the torque-producing forces are a result of both lift and drag, the blades 10, 10′ are also efficient at harnessing power from a moving fluid.

Referring again to FIG. 1, a preferred cross section is described. The cross sectional profile described below may be modified to meet anticipated fluid speeds and densities including, but not limited to, modifications to the curvatures, including the generally linear portion, the distance between leading and trailing edges, and the location of the leading edge with respect to the trailing edge, for example.

The cross section of blade 10 is configured to provide a relatively strong lift component at various wind speeds ranging from light winds to heavy winds. The cross section of blade 10 is also configured to provide a strong “cupping” or “catching” effect to maximize the amount of drag, torque-producing forces causing the blade assembly 5 to rotate in a clockwise direction, indicated by arrow “R.” The rotation direction may be reversed by altering the orientation of blades 10 and 10′. It should also be noted that the cross sectional profile for blades 10 and 10′ are identical in the preferred embodiment.

Referring to FIGS. 1 and 4, a preferred cross sectional profile is described. FIG. 4 illustrates a blade 10 which is drawn to scale. The units are preferably metric, and preferably expressed in centimeters, however FIG. 4 is a unit-less scale and the size of blade 10 may be scaled up or down, and expressed in various units, depending upon the application for which blade 10 is used. The preferred cross sectional profile has a leading edge first curved section 20. In a preferred embodiment, leading edge first curved section 20 is a portion of a circle with a substantially constant radius. In the embodiment depicted in FIGS. 1 and 4, the first curved section 20 has a non-dimensional radius of 8.5 units. Leading edge first curved section 20 has a leading edge 21 and is tangent to a substantially linear intermediate section 30 at the end of the arc opposite leading edge 21. A trailing edge second curved section 40 is also preferably a portion of a circle with a substantially constant radius. In the preferred embodiment depicted in FIGS. 1 and 4, second curved section 40 has a non-dimensional radius of 4.2 units. The second curved section 40 has a trailing edge 41 and is tangent to the substantially linear intermediate section 30 at the end of the arc opposite trailing edge 41. The linear distance along diameter 110 between the leading edge 21 and the trailing edge 41 is 21.0 units.

The preferred blades 10, 10′ depicted in FIGS. 1-4 have several advantages over previous wind turbine blade designs. The preferred blades 10, 10′ are designed to simultaneously induce a relatively strong relatively smooth lift torque-producing component as well as a relatively strong, relatively smooth drag torque-producing component and a relatively smooth counter-torque component. Counter-torque forces are inherent in Savonius-type turbines, and can be minimized, but not eliminated. The preferred blades 10, 10′ may reduce counter-torque components, for example, but not limited to, reducing the drag coefficient for the backsides 70, 70′. By keeping the counter-torque forces relatively smooth, certain embodiments introduce substantially less vibration than prior Savonius-type wind turbines. The relatively smooth torque-producing and counter-torque forces may be due to the blades's 10, 10′ cross sectional profiles, helical configuration, location with respect to one another or other factors.

Previous blade designs attempted to maximize torque or to reduce vibrations, but not to do both. The preferred blades 10, 10′ depicted in FIGS. 1-4 may have lift advantages from an asymmetrical cross sectional profile design. At the same time, the blades 10, 10′ exhibit simple curves that may be manufactured into a general helical twist along the length of the blades 10, 10′ while maintaining substantially the same cross sectional profile along the length of blades 10, 10′. The inventor recognized that previous asymmetric blade designs display complex curves across the cross sectional profile making them difficult to manufacture as a non-twisted blade, and even more difficult to form into a helix or other twist along a longitudinal axis. Previous asymmetric blade designs also exhibit a variable thickness, for example being thicker at one end and thinning towards the other end. The inventor recognized that a variable thickness may also make a blade more difficult to manufacture into a helix.

Referring to FIGS. 1 and 2, a preferred blade assembly 5 is described. FIGS. 1 and 2 are drawn to scale, and are described without limiting the preferred embodiment. The units are preferably metric, and preferably expressed in centimeters, however FIGS. 1 and 2 utilize a unit-less scale and the size of blades 10, 10′ may be scaled up or down, and expressed in various units, depending upon the application for which blades 10, 10′ are used. Many different dimensions and blade placements may be used while retaining the benefits of the described blade cross sectional profile and blade assembly arrangement. While the preferred embodiment is described with two blades 10 and 10′, additional blades, for example three or four, or more, may be used as part of other embodiments and angular offsets for additional blades will be dictated by the number of blades.

The blade assembly 5 may have a first rotor plate 200 and a second rotor plate 300. The rotor plates 200, 300 are configured to retain blades 10, 10′ in a specific relationship to one another to provide efficient and smooth wind turbine operation and to permit the blade assembly 5 to begin rotating regardless of the wind direction. For example, a groove (not shown) matching the cross sectional profile of blades 10, 10′ is formed in each rotor plate 200, 300 for each end of the blades 10, 10′. The grooves may be formed to completely traverse through rotor plates 200, 300, or may only partially traverse through rotor plates 200, 300. Other manners for holding blades 10, 10′ in place may be used, for example a suitable adhesive, or small structures formed to project from the surface of rotor plates 200, 300. Rotor plates 200 and 300 are preferably made from a rigid, durable material and are held in place and urged towards one another by various structural arrangements.

An alternate embodiment (not shown) includes a shaft (not shown) extending perpendicularly between rotor plates 200, 300. The shaft increases the structural integrity of the rotor assembly 5, but interferes with fluid flow between the blades 10, 10′. In preferred embodiments, the shaft is smaller than the gap between blades 10, 10′ and permits fluid to flow around the shaft from one blade to another. In certain embodiments, portions of leading edges 21, 21′ and trailing edges 41, 41′ that are proximate to the shaft may be connected to the shaft, for example using a tab or tie. In the preferred embodiment, there is no shaft and portions of leading edges 21, 21′ and trailing edges 41, 41′ that are centrally located, in other words, located near where a shaft would be located, may be connected to one another, for example using a tab or tie.

The leading edges 21, 21′ of the blades 10, 10′ are located at opposite ends of a diameter 110 of an imaginary circle 100. The diameter 110 has a dimension of 40.0 units. The trailing edges 41, 41′ are located on opposite sides of a second diameter 120. Second diameter 120 orthogonally bisects diameter 110. The cross sectional profile for blade 10 extends on one side of diameter 110. Trailing edge 41 is located on the same side of diameter 110 as the cross sectional profile for blade 10, and more specifically is located 1.5 units away from diameter 110. In other words, a line connecting trailing edge 41 with leading edge 21 forms an angle of approximately 4 degrees with the diameter 110. The cross sectional profile for blade 10′ extends on the opposite side of diameter 110. Trailing edge 41′ is located on the same side of diameter 110 as the cross sectional profile for blade 10′, and more specifically is located 1.5 units away from diameter 110. Depending upon design considerations, operating conditions and fluid characteristics, such as flow rate and density, for example, the distance trailing edges 41, 41′ are located from diameter 110 may be modified to be closer to or farther from diameter 110 to alter the airfoil characteristics of blades 10, 10′ as needed.

In other embodiments the leading edges 21, 21′ may be on opposite sides of diameter 110 from trailing edges 41, 41′, respectively.

Using diameter 120 as a reference, trailing edges 41 and 41′ overlap one another. Overlap is expressed in terms of the diameter of imaginary circle 100, which is 40.0 units in the preferred embodiment. Trailing edges 41 and 41′ overlap one another in a range of approximately 5 percent to approximately 15 percent. In the embodiment depicted in FIG. 1, trailing edges 41 and 41′ overlap one another by 5 percent. The blade arrangement, as well as the number of blades used, may vary depending upon prevailing wind conditions and other design considerations.

Blades 10, 10′ are preferably constructed from a lightweight, rigid material. Preferred materials include a fiber mat or sheet, made of glass or carbon fibers for example, impregnated with a resin and formed over a low density, rigid material such as acrylonitrile butadiene styrene, other plastic, foam or balsa wood, for example. Blades 10, 10′ may also be constructed from carbon composites, plastics or high density foams. Lightweight metal alloys, for example aluminum or titanium alloys may also be used to manufacture blades 10, 10′.

Blade assemblies 5 constructed according to certain embodiments employ blades 10, 10′ having a length which is greater than the width, taken across the cross section of the blade 10, 10′, in other words the distance from leading edge 21, 21′ to trailing edge 41, 41′. In preferred embodiments, blades 10, 10′ may have substantially uniform cross sections and the thickness of blade 10, 10′ may be substantially uniform. For example, blades 10, 10′ may be 0.5 to 0.8 units thick. Preferably, blades 10, 10′ are made to be as thin as possible.

Blade assemblies 5 constructed according to certain embodiments have a blade length to blade tip to blade tip diameter (in other words, the distance from leading edge 21 to leading edge 21′) ratio larger than current wind turbines. For example, the embodiment depicted in FIGS. 1-3 has a ratio of blade length to blade tip to blade tip diameter of 4:1. The 4:1 ratio is preferred, and other ratios that decrease the “footprint” of the wind turbine, for example, but not limited to, 2:1, 2.5:1, 3:1, 5:1, 8:1 or more, fall within the scope of the preferred embodiments. In other words, for a 160 centimeter blade length, a prior wind turbine requires 106.67 centimeters between the blade tips. In contrast, the described preferred embodiment requires much less distance between blade tips. For example, the embodiment depicted in FIGS. 1-3 requires only 40 centimeters between leading edges 21 and 21′ for 160 centimeter long blades 10, 10′.

By increasing the ratio for blade length to blade tip to blade tip diameter as described above, certain embodiments capture a high energy percentage from a moving fluid with a much smaller “footprint” than prior Savonius-type turbines. Wind turbines constructed according to the preferred embodiments, as well as other embodiments, require less space than prior wind turbines. Wind turbines constructed according to the preferred embodiments, as well as other embodiments, can be placed in areas where prior wind turbines do not fit. Compared to prior wind turbines, more wind turbines constructed according to the preferred embodiments, as well as other embodiments, may be placed per unit area or length, thus increasing the total amount of power harnessed from fluid moving across the area or length.

Preferred turbine blades and blade arrangements have been shown and described. While specific embodiments and applications for turbine blades have been shown and described, it will be apparent to one skilled in the art that other modifications, alternatives and variations are possible without departing from the inventive concepts set forth above. The invention is intended to embrace all such modifications, alternatives and variations as well as the preferred embodiments discussed above.

It will be obvious to those having skill in the art that many changes may be made to the details of the above-described embodiments without departing from the underlying principles of the invention. The scope of the present invention should, therefore, be determined only by the following claims. 

1. A blade for a fluid turbine comprising: an elongate body with a cross sectional profile defining a leading edge section connected to a first curved section with a first radius, a substantially linear intermediate section tangent to the first curved section, a second curved section having a second radius less than the first radius, the second curved section being tangent to the substantially linear intermediate section, and a trailing edge section connected to the second curved section; an elongate body first end and an elongate body second end where the elongate body second end is angularly and longitudinally offset from the elongate body first end so that the elongate body defines a portion of a helix; and wherein the cross sectional profile remains substantially constant over a length of the elongate body.
 2. The blade according to claim 1, wherein the first curved section is a portion of a circle with a first substantially constant radius, and the second curved section is a portion of a circle with a second substantially constant radius that is different from the first substantially constant radius.
 3. The blade according to claim 1, wherein the blade thickness is substantially uniform along the length of the elongate body.
 4. A blade arrangement for a fluid turbine comprising: a rotor plate; a first blade rigidly attached to the rotor plate, the first blade extending away from the rotor plate; a second blade rigidly attached to the rotor plate, the second blade extending away from the rotor plate; the first blade having an elongate body with a cross sectional profile defining a leading edge section connected to a first curved section with a first radius, a substantially linear intermediate section tangent to the first curved section, a second curved section having a second radius different from the first radius, the second curved section being tangent to the substantially linear intermediate section, a trailing edge section connected to the second curved section, and a first end and a second end where the second end is angularly and longitudinally offset from the first end so that the elongate body of the first blade defines a portion of a helix; and the second blade having an elongate body with a cross sectional profile defining a leading edge section connected to a first curved section with a first radius, a substantially linear intermediate section tangent to the first curved section, a second curved section having a second radius different from the first radius, the second curved section being tangent to the substantially linear intermediate section, a trailing edge section connected to the second curved section, and a first end and a second end where the second end is angularly and longitudinally offset from the first end so that the elongate body of the second blade defines a portion of a helix.
 5. The blade arrangement according to claim 4, wherein, at an end of the first and second blades attached to the rotor plate: the first blade leading edge and the second blade leading edge are located at opposite ends of a first diameter of a circle; the first blade lies on a first side of the first diameter; the second blade lies on the opposite side of the first diameter; the first blade trailing edge is located a distance from the first diameter; and the second blade trailing edge is located a distance from the first diameter.
 6. The blade arrangement according to claim 5, wherein a line connecting the first blade trailing edge with the first blade leading edge forms an angle of approximately 4 degrees with the first diameter; and a line connecting the second blade trailing edge with the second blade leading edge forms an angle of approximately 4 degrees with the first diameter.
 7. The blade arrangement according to claim 4, wherein the first blade leading edge is located on one side of a second diameter which orthogonally bisects the first diameter and the first blade trailing edge is located on the opposite side of the second diameter; and the second blade leading edge is located on the same side of the second diameter as the first blade trailing edge and the second blade trailing edge is located on the same side of the second diameter as the first blade leading edge.
 8. The blade arrangement according to claim 7 wherein the first blade trailing edge and the second blade trailing edge overlap one another by 5 to 15 percent of the distance between the first blade leading edge and the second blade leading edge.
 9. The blade arrangement according to claim 4, wherein the length of the first blade and the second blade is greater than one and one half times the distance between the first blade leading edge and the second blade leading edge.
 10. The blade arrangement according to claim 4, wherein at least a portion of the first blade trailing edge that is centrally located is connected to at least a portion of the second blade trailing edge that is centrally located.
 11. The blade arrangement according to claim 4, further comprising a second rotor plate rigidly attached to the ends of the first and second blades that are distal from the first rotor plate, and a shaft extending between the first and second rotor plates, wherein the shaft is smaller than a gap between the first blade trailing edge and the second blade trailing edge.
 12. The blade arrangement according to claim 11, wherein at least a portion of the first blade trailing edge that is centrally located is connected to at least a portion of the shaft; and at least a portion of the second blade trailing edge that is centrally located is connected to at least a portion of the shaft.
 13. (canceled)
 14. The blade arrangement according to claim 4, wherein the first blade and the second blade are configured and arranged to produce a substantially constant amount of torque-producing force and a substantially constant amount of counter-torque force for a relatively constant fluid flow.
 15. A blade arrangement for a fluid turbine comprising: means for retaining a first torque-producing means for extracting energy from a flowing fluid using both lift and drag, and for retaining a second torque-producing means for extracting energy from a flowing fluid using both lift and drag.
 16. The blade according to claim 1, wherein the leading edge section comprises a curved section.
 17. The blade arrangement for a fluid turbine according to claim 4, wherein a first blade second substantially linear section tangent to the first curved section connects the first curved section to the leading edge; and a second blade second substantially linear section tangent to the first curved section connects the first curved section to the leading edge.
 18. The blade arrangement according to claim 4, wherein, at an end of the first and second blades attached to the rotor plate: the first blade leading edge and the second blade leading edge are located at opposite ends of a first diameter of a circle and are proximate the first diameter of a circle; the first blade lies on a first side of the first diameter; the second blade lies on the opposite side of the first diameter; the first blade trailing edge is located proximate the first diameter; and the second blade trailing edge is located proximate the first diameter.
 19. The blade arrangement according to claim 18, wherein the first blade leading edge is located on one side of a second diameter which orthogonally bisects the first diameter and the first blade trailing edge is located on the opposite side of the second diameter; and the second blade leading edge is located on the same side of the second diameter as the first blade trailing edge and the second blade trailing edge is located on the same side of the second diameter as the first blade leading edge.
 20. The blade arrangement according to claim 19, wherein the first blade trailing edge and the second blade trailing edge overlap each other.
 21. The blade arrangement according to claim 20 wherein the first blade trailing edge and the second blade trailing edge overlap one another by approximately 5 percent to approximately 15 percent of the distance between the first blade leading edge and the second blade leading edge.
 22. The blade arrangement according to claim 4, further comprising a second rotor plate rigidly attached to the ends of the first and second blades that are distal from the first rotor plate, and a shaft extending between the first and second rotor plates.
 23. The blade arrangement according to claim 22, wherein at least a portion of the first blade trailing edge that is centrally located is connected to at least a portion of the shaft; and at least a portion of the second blade trailing edge that is centrally located is connected to at least a portion of the shaft.
 24. A blade for a fluid turbine comprising: an elongate body with a cross sectional profile having (a) a leading edge section having a leading edge end and an opposite end, (b) a first curved section having a first radius, the first curved section joined at a first end with the opposite end of the leading edge section, (c) a substantially linear intermediate section joined at a first end with a second end of the first curved section, (d) a second curved section having a second radius less than the first radius, the second curved section joined at a second end of the substantially linear an elongate body first end and an elongate body second end where the elongate body second end is angularly and longitudinally offset from the elongate body first end so that the elongate body defines a portion of a helix.
 25. The blade for a fluid turbine according to claim 23 wherein the cross sectional profile remains substantially constant over a length of the elongate body. 